Key Factors
Higher confidence
- Cool ‘ENSO neutral’
- Warmer-than-normal Tropical Atlantic (though not as warm as last year)
- Slightly cool eastern Pacific vs warm Atlantic creates a favourable profile for busier
Lower confidence
- Where will the MJO focus?
- African monsoon?
First of all let’s see what’s considered normal.
| Number | Named systems | Hurricanes | Major Hurricanes |
|---|---|---|---|
| 1 | Jun 20 | Aug 11 | Sep 1 |
| 2 | Jul 17 | Aug 26 | Sep 19 |
| 3 | Aug 3 | Sep 7 | Oct 28 |
| 4 | Aug 15 | Sep 16 | – |
| 5 | Aug 22 | Sep 28 | – |
| 6 | Aug 29 | Oct 15 | – |
| 7 | Sep 3 | Nov 15 | – |
| 8 | Sep 9 | – | – |
| 9 | Sep 16 | – | – |
| 10 | Sep 22 | – | – |
| 11 | Oct 2 | – | – |
| 12 | Oct 11 | – | – |
| 13 | Oct 25 | – | – |
| 14 | Nov 19 | – | – |
Some background
The first half of the June through November Atlantic hurricane season is typically quite slow with development of storms and or hurricanes forming usually within the western side of the tropical Atlantic, especially Caribbean and Gulf. Waters are warmest and ocean heat content highest towards the Caribbean. Waters further east towards Africa are cooler and atmosphere more hostile.
In the western side of the basin, the atmosphere offers more opportunity for spin ups courtesy of frontal systems dropping south out of the United States or from a phenomenon known as the CAG (Central American Gyre) which usually forms late spring and early autumn.
Due to changing winds between both ocean basins and topography of Central America, a broad low pressure circulation forms and can create small-scale low pressure to spin off the main, larger feature.
June, July typically sees the warmest, deepest waters from the Lesser Antilles westwards through Caribbean and southern Gulf.
Further east across the tropical Atlantic, waters tend to start off cool (below 27C) and so we usually don’t see much development until later July.
High concentrations of Saharan dust also plays a key role in suppressing development further east which can extend all the way to Mexico and Florida, we’ve seen this of late.
Usually once into August and September, the atmosphere is moister, less dusty and warm beyond 27C around Cape Verde and Africa’s west coast.
Current sea surface temperatures of the Atlantic. Note the cool east, warm west.
Depth of warm water (ocean heat content) shows warmest is within the Caribbean as of late May.
A more sheared and often very dusty atmosphere through the heart of the Atlantic MDR typically combines with those cooler waters in which the ‘Cape Verde season’ doesn’t typically get going until early to mid August.
In any given season, a low level AEJ (African easterly jet) forms typically mid May due to temperature differences over N Africa and can work to enhance or effect the ITCZ (inter-tropical convergence zone) which heads north of the equator. Both AEJ and ITCZ work together to enhance the west African monsoon. Embedded tropical waves or African easterly waves (AEW’s) form and ride the AEJ and this typically spawns 55-65 tropical waves or disturbances originating as far away as Somalia and the Ethiopian Highlands to drift across the continent and head out over the Atlantic.
It’s once this ITCZ lifts to typically it’s most ‘northerly position’ in combination with the AEJ and SST’s 27C+ between Caribbean and Africa where the Cape Verde season truly get’s going.
In active periods, at the height of the hurricane season and when ocean and atmosphere is primed, Africa spawns on average about 10 tropical waves which head west over the Atlantic per month. Some come to nothing while others can become closed low level circulations in the form of tropical storms, hurricanes and even major hurricanes. Some of the biggest hurricanes ever to hit the Caribbean islands, Mexico, Central America or US Gulf or Atlantic coasts have originated over the rain forests of east and central Africa.
The strength of the African monsoon is just as important as ocean temperature and wind flow above. ALL factors must come together and so we cannot just focus on whether Pacific or Atlantic is warm or cool. The MJO is another significant player in enhancing or suppressing ‘lift and convection’.
Latest weekly SST anomaly
When looking purely at Pacific and Atlantic tropical SSTA’s with slightly cool east Pacific, warm Atlantic, this configuration signals a more active season. The opposite profile (warm Pac/cool Atlantic) correlates to less active Atlantic, more active E Pac.
El Nino years and cooler tropical Atlantic (not happened for a while) is strongly LESS favourable for Atlantic but more active in the east and central Pacific.
The pendulum over the past 2 years (2024 and now 2025) has favoured a more active Atlantic compared to Pacific due to La Nina and now neutral ENSO but with a current cool La Nina hangover likely to hold through the upcoming season.
Neutral to cool phases of ENSO show a busier Caribbean and Gulf compared to El Nino.
Based purely on SSTA profiles, while this year continues to favour Atlantic over Pacific, it’s less cool in Pacific and warm in Atlantic but still lean towards busier.
Last year
This year
While the latest SSTA’s show one thing, what about the June-November season itself? Models support a similar profile through the season.
Here’s the North American multi-model ensemble for SSTA’s June-August.
August-October
Role of the Maddan Julian Oscillation
Put simple, the MJO is an eastward progressing area of ‘enhanced convection’ circumnavigating the tropics. This plays a crucial role in determining where experiences upward motion (low pressure) and where sinking (high pressure).
This large-scale atmospheric wave of enhanced showers and thunderstorms circumnavigates the tropics, typically completing it’s journey within 30-60 days.
SSTA’s can determine where the MJO focuses it’s active and inactive phases, often lingering for for weeks in a given region. La Nina and El Nino as well as the Indian Ocean Dipole can dictate phases of the MJO.
The MJO can enhance and squash the Asian and African monsoons and global tropical activity.
The influence of the MJO is determined by how strong or weak (amplified) it is in a given region and when strong and is over say the Indian Ocean, it usually supports an uptick in the Asian monsoon from India to Malaysia and tropical cyclone activity at the expense of a quieter Pacific and west Atlantic due to these basins under the downward (sinking) branch. The opposite is true when the rising branch enters the east Pacific and Atlantic. Sometimes a easing of the Asian monsoon intensity, less cyclones in the Indian Ocean/West Pacific and an uptick further east over the Pacific and Atlantic.
Where the convection is enhanced, the jet stream strengthens to the north and also affects the strength, position or lack thereof the ITCZ.
It’s important to note that while in a ‘highly amplified state, can have huge influences, sometimes it’s weak and bares little influence.
ODDITIES OF 2024: Despite Seemingly Perfect Conditions, There’s No Guarantee Of A Fast & Busy Season…
Last year in the heart of the season (August), the MJO was strong over the Indian Ocean helping drive a strong SW Asian monsoon but in turn while there was large-scale rising motion over the eastern hemisphere, the downward branch (sinking) was focused on the Atlantic which significantly suppressed the ability for AEW (African easterly waves) to develop. Also, the position of the ITCZ was in fact unusually far north, meaning less robust waves as they crossed the arid southern end of the Sahara which caused greater quantities of dust to sweep out over the Atlantic.
Colder waters nearer the latitude of the Canary Islands also played a role in the long lived hiatus in tropical development. Could this occur again in 2025? I believe the reason behind the unusually far north convergence zone was a northward shift in the subtropical high.
Let’s take a look at the models regarding vertical velocity and precipitation distribution.
CFSv2
The greens represent upward motion and the reds sinking. The CFSv2 shows rising over West Pac/Central America in June which could suggest a few named storms, especially given how warm the waters are at depth.
July
July suggests a stronger amplitude with rising over much of the Pacific (esp west) and consequential sinking over the Atlantic, suggesting less activity.
August
Even August isn’t particularly favourable looking though it has to be said, this is very far out and likely to chance.
Of course the heart of the season doesn’t come until mid-August so here’s the 3-month vertical velocity for August-Sep-Oct, the 3 most important months combined.
Again not particularly enthusiastic. Let’s remember, this is ONE model and there are many with differing solutions.
As for precip, let’s look at the NMME for each month June through October.
Again, take with a large pinch of salt as this is ONE model of many and looking far out into the future. However, the combo of the vertical velocity which shows rising (lower pressure/higher rainfall) and sinking (higher pressure/less rain), but the precip outlook quite closely reflects one another and it favours an active Asian monsoon and EAST Africa monsoon, less West Africa which is important in terms of potential AEW production and the amount transiting westwards over the Atlantic.
Strength and position of the Mid Atlantic ridge is also a factor to consider. Which is strongest (Bermuda or Azores). The cooler waters over the east side of the tropical Atlantic could support a strongest Azores high which in turn may create more of a weakness over the central Atlantic which opens a door to more systems turning north and missing land areas further west.
The Verdict…
With ALL the above factors taken into account, I believe we have another active and above normal hurricane season ahead with potential of early development over the western Atlantic thanks to an active MJO in the coming weeks.
As is often the case, the season will be stop-start depending upon the MJO and whether the Atlantic is under the rising or sinking branch. We could see some pretty rapid spin ups, especially over the higher ocean heat content of the western basin including Caribbean and Gulf which I think will become a magnet for moderate to high end hurricanes (Cat 2-4) helped by low shear and waters 2-3C above norm.
I also think, we’ll have a near average number of long track African waves which may be slower to develop over the slightly cooler eastern MDR. It’s much cooler compared to last year. So, while the wave train appears normal, throwing out 7-10 waves per month, development could be slow or nonexistent over the east and central basin.
We may see ‘spates’ of formation, perhaps 2 or even 3 in June, then a lengthy quiet period later June and through a large chunk of July with possibly higher than usual concentrations of Saharan dust.
Of course, it’s guesswork predicting where and when systems will develop but I suspect the basin will liven up as August progresses with peak activity as per normal late August through September but again this depends upon atmospheric conditions and MJO.
The cool east Pacific/warm west Atlantic favours busier and with a concentration of systems developing and perhaps become strong (Cat 3+) within the Caribbean and Gulf. We’ll see potentially an even spread of named systems heading west as well as turning north into the open N Atlantic.
Here’s my numbers.
Areas with a moderate to high risk of impact from storms and or hurricanes.
As you can see I’ve highlighted a large ‘high risk area’ due to very warm waters and the fact that there’s strong evidence from the past supporting a busy Caribbean when waters are warmer than normal in this region vs cooler in the East Pac. Another high risk is the central US Gulf Coast from the TX-LA border to far west FL panhandle with moderate risk of impacting storms south Texas below Houston and the Carolinas.
Featured image credit: By NASA/Goddard Space Flight Center – Dramatic Views of Hurricane Florence from the International Space Station From 9/12, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=155163327
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